STORY HIGHLIGHTS

Usually first presidential debates expose a candidate to a public beyond their home state or the Washington political bubble

Hillary Clinton and Donald Trump came out swinging in a crunch US presidential debate today, with the Republican and Democrat aggressively pitching their case to tens of millions of viewers.

Clinton hit her rival as coming from a "very fortunate" background and called on voters to judge them both on the basis of "who can shoulder the immense, awesome responsibilities of the presidency."

Trump ditched his red power tie for a more statesmanlike blue and took on a restrained tone, but reprised the themes that have catalysed his improbable campaign, insisting jobs were being lost to Mexico and China.

"We have to renegotiate our trade deals and we have to stop these countries from stealing our companies and our jobs," he said, saying Clinton had decades without making lives better for ordinary Americans.

Clinton hit back, accusing Trump's economic platform of amounting to "the most extreme" package of tax cuts for the wealthy in US history.

"I call it trumped up trickle-down, because that's exactly what it would be," she quipped.

This first 2016 presidential debate could be pivotal in deciding whether Clinton will become the first woman president, or if Trump can pull off the greatest upset in US political history.

When the celebrity businessman launched his campaign in June 2015, bookmakers put his odds at 100/1 and he was roundly mocked.

But the 70-year-old weathered allegations of bigotry and sexism to triumph in a vicious Republican primary campaign.

He now has a real shot at being sworn in as the 45th president of the United States on January 20.

Six weeks out from election day the polls have tightened to a virtual dead heat.

Clinton, the 68-year-old former secretary of state, first lady and US senator, is perhaps the most qualified presidential candidate since George Bush senior, Dwight Eisenhower or Ulysses Grant.

She has a massive organisational advantage, a bigger campaign warchest, a lead in the popular vote and is in a notably stronger position state-by-state.

But she remains deeply unpopular. In a country split 50/50 between Democrats and Republicans, only 40 per cent of voters say they have a favorable view of her.

Polling group Gallup reports that 69 per cent of US adults are sure they will vote, down from 76 per cent in 2012.

In the last two presidential elections, young, black, Hispanic and Asian-American voters turned out in record numbers for Barack Obama.

Clinton's campaign has struggled so far to match that level of enthusiasm.

Only 47 per cent of voters aged 18 to 34 say they will definitely vote this time round, that is down from 74 per cent when Obama was first elected.

This Super Bowl of politics is taking place at Hofstra University on Long Island, a mere 60-minute drive from Manhattan and chaired by NBC Nightly News anchor Lester Holt.

The questions will revolve around three themes: "America`s direction, achieving prosperity, and securing America."

Watching from the venue were Melania Trump and Bill Clinton, two spouses who have made their impact felt on during the campaign.Monday`s target will be the estimated nine per cent of American voters who are still undecided. Can the Democrat win their minds, if not their hearts? Can Trump persuade them that he has the gravitas to lead?

If Trump remains disciplined and focuses on issues, he could earn credibility among college-educated whites.

Clinton will no doubt aim to seize on Trump`s toxic campaign trail rhetoric, using his words to paint her rival as a divisive demagogue.

But she may struggle to meet expectations. A Quinnipiac poll found that voters expect Clinton to win the debate 41 to 32 per cent, and that 84 per cent said they intend to watch.

Clinton's campaign has expressed concerns of being held to a double standard, saying the bar is higher for her while Trump stood to win praise for a merely adequate performance.

Usually first presidential debates expose a candidate to a public beyond their home state or the Washington political bubble. But like much else in the 2016 race, that is not the case this year.

The names of both candidates were recognised by almost 100 per cent of the adult population, even before the debate.